U.S. patent number 5,304,330 [Application Number 07/748,032] was granted by the patent office on 1994-04-19 for preparation of mixed fiber composite structures.
This patent grant is currently assigned to Auburn University. Invention is credited to David A. Kohler, Gopal A. Krishnagopalan, Millard F. Rose, Bruce J. Tatarchuk, John N. Zabasajja.
United States Patent |
5,304,330 |
Tatarchuk , et al. |
April 19, 1994 |
Preparation of mixed fiber composite structures
Abstract
A new class of composites results from a matrix of fibers, such
as fibers of carbon, alumina, ceramics, and aluminosilicates,
interwined in a network of fused metal fibers. The composites can
be fabricated to have varying surface area, void volume, and pore
size while maintaining high electrical conductivity. Composites are
readily prepared from a preform of a dispersion of the metal
fibers, other fibers, and an organic binder such as cellulose, by
heating the preform at a temperature sufficient to fuse the metal
fibers and to volatilize at least 90% of the binder. Where a carbon
fiber is used, the metal fibers are fused at a temperature causing
a loss of less than about 25%, and usually under 15%, by weight of
carbon fiber.
Inventors: |
Tatarchuk; Bruce J. (Auburn,
AL), Rose; Millard F. (Auburn, AL), Krishnagopalan; Gopal
A. (Auburn, AL), Zabasajja; John N. (Baton Rouge,
LA), Kohler; David A. (Baton Rouge, LA) |
Assignee: |
Auburn University (Auburn
University, AL)
|
Family
ID: |
26999401 |
Appl.
No.: |
07/748,032 |
Filed: |
August 21, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
356861 |
May 24, 1989 |
5080963 |
|
|
|
435167 |
Nov 13, 1989 |
5102745 |
|
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|
Current U.S.
Class: |
419/2; 264/104;
264/122; 264/DIG.75; 264/125; 419/11; 264/105 |
Current CPC
Class: |
H01G
11/72 (20130101); D04H 1/4234 (20130101); B22F
3/002 (20130101); D04H 1/4242 (20130101); D04H
1/4209 (20130101); H01G 11/58 (20130101); H01G
9/155 (20130101); D04H 1/43835 (20200501); D04H
1/43838 (20200501); Y02E 60/13 (20130101); Y10S
264/75 (20130101) |
Current International
Class: |
B22F
3/00 (20060101); D04H 1/42 (20060101); H01G
9/00 (20060101); C04B 035/64 () |
Field of
Search: |
;264/61,104,105,122,125,65,56,59,60,29.4,29.6,29.2,248,DIG.75,257
;428/288,296,605 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Woo; Jay H.
Assistant Examiner: Smith; Duane S.
Attorney, Agent or Firm: Snyder; Eugene I.
Government Interests
ACKNOWLEDGMENTS
This work was funded by Auburn University and the Space Power
Institute as funded by the SDIO Innovative Science and Technology
Office and the Defense Nuclear Agency under DNA contract no.
001-85-C-0183.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of my copending
applications, Ser. No. 07/356,861, filed May 24, 1989 now U.S. Pat.
No. 5,580,963, and Ser. No. 07/435,167, filed Nov. 13, 1989 now
U.S. Pat. No. 5,702,745, all of which are hereby incorporated by
reference.
Claims
What is claimed is:
1. A method of making a composite comprising the steps of forming a
dispersion comprising carbon fibers, metal fibers, and cellulose in
an unreactive liquid medium, collecting the resulting wet
dispersion, removing the unreactive liquid medium from the wet
dispersion to afford a dried preform, heating the dried preform in
an atmosphere containing hydrogen at a temperature effective to
volatilize at least 90 weight percent of the cellulose and fuse the
metal fibers, with a weight loss of under about 25 weight percent
of carbon fibers, and recovering the resultant composite.
2. The method of claim 1 where the metal fiber is selected from the
group consisting of aluminum, titanium, vanadium, chromium, iron,
cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum,
ruthenium, rhodium, palladium, silver, cadmium, indium, tin,
hafnium, tantalum, tungsten, rhenium, osmium, platinum, gold,
antimony, beryllium, iridium, silicon, magnesium, manganese,
gallium, and combinations thereof.
3. The method of claim 1 where the metal fiber is an alloy.
4. The method of claim 3 where the alloy is selected from the group
consisting of constantan, hastelloy, nichrome, inconel, monel,
carpenter's metal, steels, and non-steel iron alloys.
5. The method of claim 4 where the alloy is a stainless steel.
6. The method of claim 1 where the cellulose is at least 95% weight
percent volatilized.
7. The method of claim 6 where the cellulose is at least 99 weight
percent volatilized.
8. The method of claim 1 where under about 15 weight percent of the
carbon fibers are lost.
9. The method of claim 8 where under about 5 weight percent of the
carbon fibers are lost.
10. The method of claim 1 where the cellulose is at least 99 weight
percent volatilized at temperatures sufficient to fuse the metal
fibers with a loss of less than about 25 weight percent carbon
fibers.
11. The method of claim 1 where the cellulose is at least 99 weight
percent volatilized at temperatures sufficient to fuse the metal
fibers with a loss of less than about 5 weight percent carbon
fibers.
12. A method of making an article having a network of a first fiber
and at least one second fiber, where at least said first fiber has
a plurality of bonded junctions at the first fiber crossing points
and said second fiber is interlocked in the network of bonded first
fibers, said method comprising the steps of forming a dispersion in
an unreactive liquid, said dispersion comprising the first and the
second fibers, and at least one structure forming agent selected
from the group consisting of cellulose, polyvinyl alcohol,
polyurethanes, styrene-butadiene latex, epoxy resins,
urea-formaldehyde resins, and polyamide-polyamine epichlorohydrin
resins, collecting a wet dispersion, removing the unreactive liquid
from the wet dispersion to afford a preform, treating the preform
to effect bonding of at least the first fibers at a plurality of
the first fiber junctions, removing at least 90 weight percent of
the structure forming agent, and recovering the article.
13. The method of claim 12 where the first fiber is a metal and the
second fiber is selected from the group consisting of a metal, a
ceramic, a high surface area material with a surface area from 1.5
to about 1500 square meters per gram, carbon, or any combination
thereof.
14. The method of claim 13 where the second fiber has a surface
area from 1.5 to about 1500 square meters per gram and is selected
from the group consisting of silica, carbon, magnesia, alumina,
titania, aluminosilicates, aluminophosphates, and
silicaaluminophosphates.
15. The method of claim 14 where the second fiber is impregnated
with a third metal, said third metal selected from the group
consisting of aluminum, titanium, vanadium, chromium, iron, cobalt,
nickel, copper, zinc, zirconium, niobium, molybdenum, ruthenium,
rhodium, palladium, silver, cadmium, indium, tin, hafnium,
tantalum, tungsten, rhenium, osmium, platinum, gold, antimony,
berrylium, iridium, silicon, magnesium, manganese, and gallium.
16. The method of claim 13 where the second fiber has a surface
area of at least 50 square meters per gram.
17. The method of claim 13 where each of first and second fibers in
a metal and where each fiber is:
1) a metal selected from the group consisting of aluminum,
titanium, vanadium, chromium, iron, cobalt, nickel, copper, zinc,
zirconium, niobium, molybdenum, ruthenium, rhodium, palladium,
silver, cadmium, indium, tin, hafnium, tantalum, tungsten, rhenium,
osmium, platinum, gold, antimony, berrylium, iridium, silicon,
magnesium, manganese, and gallium.
2) any combination of said foregoing metals;
3) an alloy of at least two of said metals; or
4) any combination of said metals and said alloys.
18. The method of claim 17 where the second fiber has a diameter
from about 0.001 to about 1000 times that of the first fiber.
19. The method of claim 17 where the second fiber has an aspect
ratio between about 10 and about 10,000.
20. The method of claim 17 where the second fiber is present at a
weight ratio of from about 0.001 to about 100 that of the first
fiber.
21. The method of claim 13 where the second fiber is a ceramic
selected from the group consisting of the oxides, carbides, and
nitrides of aluminum, titanium, vanadium, chromium, iron, cobalt,
nickel, copper, zinc, zirconium, niobium, molybdenum, ruthenium,
rhodium, palladium, silver, cadmium, indium, tin, hafnium,
tantalum, tungsten, rhenium, osmium, platinum, gold, antimony,
berrylium, iridium, silicon, magnesium, manganese, and gallium and
mixtures thereof.
22. The method of claim 12 where at least the first fiber is
electroplated.
23. The method of claim 12 where at least the first fiber has a
conductive coating.
24. The method of claim 12 where the first fiber is a metal and the
second fiber has a surface area of from 1.5 to about 1500 square
meters per gram.
25. The method of claim 24 where the second fiber is selected from
the group consisting of silica, carbon, magnesia, alumina, titania,
aluminosilicates, aluminophosphates, and
silicaaluminophosphates.
26. The method of claim 12 where the preform is treated so as to
remove at least 95 percent of the structure forming agent.
27. The method of claim 12 where the procedure of treating the
preform to effect bonding is selected from the group consisting of
heating, electroforming, electroplating, chemical vapor deposition,
and reactive plasma spraying.
28. The method of claim 12 further wherein said preform is
compacted under pressure prior to being treated to effect
bonding.
29. The method of claim 12 where the first fiber is:
1) a metal selected from the group consisting of aluminum,
titanium, vanadium, chromium, iron, cobalt, nickel, copper, zinc,
zirconium, niobium, molybdenum, ruthenium, rhodium, palladium,
silver, cadmium, indium, tin, hafnium, tantalum, tungsten, rhenium,
osmium, platinum, gold, antimony, berrylium, iridium, silicon,
magnesium, manganese, and gallium.
2) any combination of said foregoing metals;
3) an alloy of at least two of said metals; or
4) any combination of said metals and said alloys.
Description
BACKGROUND OF THE INVENTION
This application relates to novel composites having high surface
area, variable porosity and void volume, good conductivity, and
displaying chemical stability in corrosive environments. The
composites have mechanical and structural integrity and can be
prepared in virtually an endless variety of shapes. For the sake of
simplicity and clarity of exposition, the composites which are our
invention will be discussed from the aspect of their use as
electrode materials. However important this particular application
may be, it needs to be stressed at the outset that the claimed
composites have significant utility outside the field of
electrochemistry, but such uses are omitted here only for
convenience.
Carbon based electrodes are currently used in many high energy
density and/or high power density applications, such as
Li/SOCl.sub.2 batteries, liquid double layer capacitors, and fuel
cells. The maximum energy and power densities obtainable from these
devices depend upon various physicochemical rate phenomena
occurring at the electrode-electrolyete interface. For example, in
the case of high energy density lithium/thionyl chloride batteries,
deactivation of the carbon cathode limits operation of the battery
at high (>10 mA/cm.sup.2) discharge rates. Since deactivation
arises from the preferential precipitation of solid reaction
products at the exterior of the cathode, thereby blocking its
interior surface area from participating in the reaction, the power
density of the battery during discharge is limited by the porosity,
the void volume, and the active or accessible surface area of the
carbon cathode.
When the cathode becomes blocked, as described above, the
interfacial electrochemical reaction of the anode becomes limited
by the dissolution rate of the reaction products into the
electrolyte, which in turn is controlled by the precipitation rate
at the cathode. Attempts to improve the fabrication and design of
the carbon cathode has had limited success. Much of this activity
has involved the addition of metallic elements such as copper to
the carbon or the coating of the cathodes with transition metal
phthalocyanines. Other efforts have utilized various carbon
pretreatment procedures or different types of carbon blacks with
various physical properties. However, past attempts appear not to
address the intrinsic problem associated with carbon blacks, viz.,
the inaccessibility of small pores within the microstructure of the
material and the existence of low void volumes in the outermost
layers of the carbon. To provide high power density cathodes what
is needed are materials which are flexible, have high specific
surface areas, have varying and adjustable porosities and void
volumes to accommodate reaction products as precipitates without
significant loss of surface area, and which are corrosion
resistant.
In liquid double layer capacitors the energy density increases with
increased active surface area of the electrode presented to the
electrolyte, whereas the power density is controlled and limited by
slow diffusion of electrolyte through the microporous electrode
material. To increase both the energy and power density of these
capacitors requires increased diffusion processes, which prefer
large pores and high void volumes, and higher levels of specific
surface area, which entails small pore sizes and low void volumes.
To date the requirements of large pores/high void volume and high
surface area tend to be mutually exculsive. Consequently, since
increased energy density involves increased surface area and
increased porosity, power dense devices become more and more
limited by diffusion processes as the surface area of the electrode
is increased.
In fuel cells, an effective electrode material should exhibit high
catalytic activity and high electrical conductivity to minimize
joule losses within the device. The electrode should be highly
porous to provide free access to both the gases and the
electrolytes. The optimum pore size distribution of the electrode
material is a compromise between several factors. For high
strength, low porosity and small pores are desirable. For low
polarization, large pores with maximum internal surface area are
more desirable. Electrodes also contain metals such as platinum,
nickel, and so forth, which are good catalysts for fuel oxidation
and oxidant reduction. The catalytic activity depends on the active
surface area of electrode as well as the contacting of the
electrode with reactants consisting of fuel and electrolyte. For
this reason, controlled wetting of the electrode poses one of the
more severe design limitations confronting the device in order to
provide optimal contacting at the gas-liquid-catalyst interface in
the absence of weeping, bubbling, and flooding.
Carbon is an especially attractive electrode material, and high
surface area carbon electrodes typically are fabricated from
activated carbon blacks. However, a major difficulty in fabricating
and utilizing high surface area carbon electrodes has been in
physically supporting the carbon. Carbon black usually is used in
the powdered form which cannot be easily supported unless
poly(tetrafluoroethylene) (PTFE) or other types of binders are
used. Our radically different approach has been to combine
dissimilar and normally incompatible materials to form a physically
stable composite structure which exhibits properties intermediate
to the constituent materials. In the context of carbon electrodes,
the resulting materials have a high surface area, variable porosity
and variable void volume, are structurally stable, and can be
fabricated in a virtually endless variety of shapes and sizes. More
particularly, high surface area carbon fibers and highly conductive
metal fibers have been combined in an intertwined sinter-locked
network or grid which is structurally stable. The resultant high
surface area and conductive composite allows high accessibility to
gases and electrolytes while providing adjustable porosities and
void volumes. Interlocked networks of thin fibers can be bonded to
metallic backings, serving as current collectors and bipolar
electrolyte separators, to provide flexible electrode structures
which can be readily assembled into devices even when one of the
components is relatively brittle or does not normally bond or
adhere to the metal backing.
A generic approach to high surface area has been to disperse carbon
blacks in an organic resin which serves as a mechanical framework.
Solomon in U.S. Pat. No. 4,500,647 exemplifies one approach by
using a matrix of carbon particles within an unsintered (i.e.,
unfused) network of carbon black-filamentary PTFE. The use of PTFE
as a matrix for carbon particles has been investigated extensively.
However, the addition of PTFE reduces the electrical conductivity
of the cathode active layer (Solomon et al. in U.S. Pat. Nos.
4,500,647, 4,518,705, 4,456,521) and the cost of using PTFE has led
others to seek alternative means of holding the carbon black
together (Aubrey D. Smith, National Technical Information Service
Technical Note, Report Date-Feb. 1986, 1 page, NTN86-0166).
A somewhat different approach employs carbon particles in a
carbonized matrix. For example, Christner et al. (U.S. Pat. No.
4,115,528) prepared a porous carbon sheet by coating carbon fibers
with furfuryl alcohol and a catalyst effective for its
polymerization. The mixture was formed into a mat, heated to effect
polymerization, and the resin then was further heated to carbonize
the resin. The patentee in U.S. Pat. No. 4,506,028 dispersed carbon
fibers in an organic binder containing organic poreforming
granules, then heated the mix to carbonize the binder and
volatilize the poreformers. In both of the foregoing the carbonized
matrix supplies structure (i.e., rigidity and mechanical strength),
whereas in our composites structure is afforded by a grid of fused
metal fibers.
Zuckerbrod et al. in U.S. Pat. No. 4,448,856 describes an electrode
with a layer of paste consisting of carbon particles, stainless
steel particles, a fluorinated polymeric binder, and a catalyst for
decomposition of peroxides. It is noteworthy that such a paste must
contain at least 20 weight percent stainless steel relative to
carbon particles. Finally, Watanabe and coworkers, [J. Electrochem.
Soc: Electrochemical Science and Technology, 134, 1318 (1987)] used
polyethylene glycol as a binder for carbon blacks, then pressed a
film of the resulting material on nickel wire for use as a cathode
in a lithium cell. It may be mentioned in passing that
electrochemical electrodes have been described in U.S. Pat. No.
3,905,831 consisting of a pile fabric where a portion of the yarn
is metallic. The patentees mention that the metal fibers in the
yarn may be bonded, as by sintering, brazing or welding.
However useful and significant the carbon fiber-metal fiber network
previously referred to may be, it seems to us that it is but one
example of a class of composites with a range of uses transcending
those of electrochemical applications and encompassing such diverse
areas as cellular supports in biochemical reactors, magnetic
separators, and filters; a short exposition of some of these uses
is deferred to a later section. As to the composites themselves, it
appears to us that one can specify their lowest common denominator,
that is, those irreducible features which are necessary and
sufficient to impart to the class of our composites those
characteristics which make the class desirable from a materials
point of view. A necessary feature is that the composite be a
network of at least two different fibers. The fibers could be
chemically different, for example, a metal fiber and a carbon
fiber, or they could be physically different, for example, two
fibers of the same metal but with different cross-sectional
dimensions, length, or aspect ratio. The second and only other
necessary feature is that there be a number of points in the
network where the fibers are physically connected, i.e., bonded.
There is versatility and variability here, too, such as the
relative number of bonded points, whether fibers "interbond" (i.e.,
bonding between dissimilar fibers), whether they only "intrabond"
(i.e., bonding between similar fibers), and if there is
intrabonding whether all classes of fibers so bond or whether only,
say, one kind of fiber bonds. The resultant composite is then a
network of at least a first and a second fiber where the second
fiber is interlocked in the network of bonded (i.e., physically
connected) first fibers.
A pictorial, somewhat fanciful, and certainly non-literal overview
of our invention is depicted in FIG. 1. The left hand region,
designated by A, represents a physical mixture of two kinds of
fibers as shown by the open and dotted strands. The case where only
one of these fibers is intrabonded is depicted by B, that where
both kinds of fibers are intrabonded is depicted by C, and that
where the fibers are interbonded is depicted by D. The relative
amounts of the two fibers will quite obviously influence the void
volume of the composite. The density of bonded points will affect
structural flexibility and, where the bonded fibers are
electrically conducting, the conductivity of the composite. Where
one fiber is non-porous, the relative number of the two fibers will
determine the porosity of the composite. In short, from this
oversimplified pictorial representation one can easily see how the
final properties of the composite can be varied and one can
appreciate that the properties of the composite can be a blend of
the properties of dissimilar, normally incompatible materials--that
is, the properties of the composite are themselves a composite of
the properties of the materials forming the network. This attribute
can not be stressed too highly since it is, if not unique, rarely
found, difficult to achieve, and highly desirable for new
materials.
SUMMARY OF THE INVENTION
The purpose of our invention is to provide as new materials
composites formed from dissimilar fibers, composites whose
physicochemical characteristics may be the resultant of the
physicochemical characteristics of the dissimilar fibers present,
whose physicochemical characteristics can be varied, and whose
properties are under the control of the fabricator. More
particularly, the purpose of the invention to be described within
is the preparation of such composites. An embodiment comprises
dispersing a first fiber, at least one second fiber, and a
structure forming agent in a liquid, casting the dispersion into a
preform, and treating the cast dispersion to effect bonding of at
least the first fibers at a plurality of their junctions. In a more
specific embodiment the structure forming agent is a cellulose. In
a still more specific embodiment the cast dispersion is heated to
sinter-lock at least the first fibers and to volatilize at least
90% by weight of the structure forming agent. In yet another
embodiment the first fibers are metals and the structure forming
agent is cellulose. Other purposes and embodiments will be apparent
from the description which follows.
DESCRIPTION OF THE FIGURES
FIGS. 1A-D are a pictorial and non-literal overview of the
invention.
FIG. 2 depicts the assembly of paper preforms into electrode
preforms prior to sintering.
FIG. 3 is an electronmicrograph of a stainless steel-cellulose
composite paper before sintering, where the stainless steel fibers
are 2 microns in diameter.
FIG. 4 is an electronmicrograph of a stainless
steel-carbon-cellulose composite paper before sintering; stainless
steel fibers are 2 microns in diameter.
FIG. 5 is an electronmicrograph of the stainless steel-carbon
composite matrix after sintering the composite paper of FIG. 4 at
conditions of Experiment E in Table I.
FIG. 6 is an electronmicrograph of the stainless steel-carbon
composite of FIG. 5 at higher magnification showing intimate
contacting of metal and carbon fibers after sintering.
FIG. 7 is an electronmicrograph at still higher magnification of a
metal-metal joint after sintering.
FIG. 8 is an electronmicrograph of a metal fiber composite
entrapping fibers of 95% alumina-5% silica; see Example 2.
FIG. 9 is an electronmicrograph of a metal fiber composite with an
entrapped mica platelet and kaolinite particles chemically attached
to the stainless steel fibers; see Example 4. FIG. 10 is the same
sample at higher magnification.
FIG. 11 is an electronmicrograph of a composite of metal fibers
with two different diameters having entrapped within the network a
biosupport. FIG. 12 is an electronmicrograph of the same composite
which has been impregnated with growing cells; see Example 5.
FIG. 13 shows polarization curves for the reduction of oxygen in an
alkaline fuel cell; see Example 8.
DESCRIPTION OF THE INVENTION
We have found a generalized method of making composite articles
which is enormously versatile both with respect to the materials of
the resulting composite as well as the shape of the resulting
article. Our method bonds fibers at a plurality of their junctions
in a fibrous network, which inter alia has the effect of imparting
high strength and structural integrity to the fibrous network and
to afford good electrical contact when one of the fibers is a metal
or another type of conductive, or conductively coated, material.
One result of our method is that it is possible to combine
dissimilar materials with dissimilar properties where the materials
and properties often are considered incompatible or mutually
exclusive, and to obtain an article having mutually beneficial
properties characteristic of each of the dissimilar materials.
Because of the broad operability of our method when applied to a
wide spectrum of materials, we have examined the resulting articles
of manufacture as to their properties, as to their several uses,
and as to the variants which can be expected based on our
experience. In one aspect, then, our invention is a generalized
method of making articles, and especially shaped articles,
containing at least two kinds of fibers bonded in a network. The
lowest common denominator of these articles, that is, the feature
which is common to each of them, is a network of at least two
classes of fibers where the fibers from at least one of the classes
are bonded at a multiplicity of junctions within the network and
the fibers from the other classes are interlocked in the network of
bonded fibers. This theme will be elaborated upon in greater detail
below.
We use the term "network" in the usual dictionary definition, i.e.,
a structure of [cords or wires] that cross at regular intervals and
are knotted or secured at the crossing. See Webster's Seventh New
Collegiate Dictionary, G. and C. Merriam Co., (1970), p. 568. We
note that the networks of our invention are two dimensional in the
sense that the flat-shaped article has a thickness which is often
small relative to the other lateral dimensions. However, the
diameter of the largest size fiber also will be small relative to
the thickness of the article, which means that the articles of our
invention are not composed of a monolayer of fibers but instead are
composed of multiple layers of discrete fibers. In the context of
the definition of a network, the crossing points of the fibers may
be in different planes, and it follows that the fibers will not be
in contact at all crossing points. In this application "junctions"
refers to the crossing points in the network where fibers are in
contact or caused to come into contact.
One can readily exemplify and illustrate some important properties
of our composites in the sphere of electrodes where often it is
desired that materials have a high surface area, high void volume,
and high electrical conductivity. Although it is not necessary that
all three attributes be manifested simultaneously in every physical
device utilizing an electrode, it would be quite desirable to have
an electrode material which not only permits variability in these
properties, but also affords the option of preparing an electrode
with that set of properties optimum for a particular application.
Properties such as high surface area and high electrical
conductivity tend to be mutually exclusive. This situation arises
because, for example, carbon has a low density (relatively high
surface area) and modest conductivity whereas metals have a high
density (relatively low surface area) accompanied by a generally
high conductivity. Consequently, the properties of prior art
materials, and in particular the mutual exclusivity of two of the
three properties given above, restrict the set of simultaneously
attainable properties available and preclude the option of complete
design manipulation.
Conceptually a marriage of carbon and metals might result in a
composite with the best features of both. However, carbon blacks
and metals do not form strongly adhesive bonding arrangements with
each other and possess quite different densities and tensile
properties. Consequently they do not mix well when dry nor provide
good adherence to metal substrates under normal conditions. As
previously stated, our goal has been to combine dissimilar and
normally incompatible materials to form a physically stable
composite structure which exhibits properties that are intermediate
to the constituent material. This goal is achieved successfully in
composites which are a matrix of carbon fibers intertwined and
interlocked in a network of fused metal fibers.
As previously stated, the feature common to all articles of our
invention is a network of at least two classes of fibers where at
least one class is bonded at a plurality of their junctions and the
fibers from the other classes are interlocked in the network of
bonded fibers. By "bonded" is meant that the fibers are physically
connected, either directly or via a link or bridge between the
fibers. In particular, "bonded" does not include mere physical
contact of two fibers but rather requires some sort of permanent
union or "gluing together" of the fibers; bonded fibers are
securely connected, locked together. Note also that two fibers can
be bonded without their being in direct physical contact, but with
indirect contact provided by a link or bridge between them. Whether
only the first fibers are bonded at their junctions or whether both
the first and the second fibers are bonded at their junctions
depends on the materials of the article, the bonding method, and
the bonding conditions. Similarly, the question whether the first
and second fibers are bonded to each other also depends on the
fibrous materials, the bonding method, and bonding conditions. For
example, where both the first and second fibers are of the same
metal, then generally both the first fibers will be bonded to each
other at their junctions and the second fibers will be bonded to
each other at their junctions, as well as the first and second
fibers being bonded to each other at their junctions. Where, for
example, the second fiber is a metal which is dissimilar from that
of the first fiber, and where the bonding method is sintering, then
the question of whether the second fiber will be bonded at its
junctions will be dependent upon sintering temperature and
sintering time as well as the particular metal constituting the
second fiber. Similar considerations apply to the question whether
the first and second fibers will be bonded at their junctions. In
contrast, where the first fiber is a metal and the second fiber is,
for example, a ceramic, the kind of bonding will be quite dependent
upon the particular bonding method and bonding conditions. So, for
example, where heating is the bonding method then at sufficiently
high temperatures to sinter both the metal and the ceramic the
first fiber will be bonded at its junctions and the second fiber
bonded at its junctions, but generally there will be no bonding at
the junctions of the metal and the ceramic.
On the other hand, where the first fiber is a metal and the second
fiber a nonmetal, if electroplating is the bonding method and if
the second fiber can be electroplated under the conditions
employed, then there would be bonding at all junctions. Where only
the metal is electroplated under the conditions used then only the
junctions of the first fiber will be bonded. For example,
electrodeposition of nickel onto or into the metal fibers in a
network of 2 micron diameter stainless steel and 2 micron diameter
carbon fibers causes a physical enlargement of the metal fiber
diameters which leads to an increase in the electrical and physical
contact between carbon and metal by greater than 30%. Such a
procedure provides one example whereby bonding between dissimilar
materials can be enhanced for desired electrical properties or
other favorable mechanical attributes. In any event, the question
of what junctions are bonded generally can be answered from a
knowledge of the materials used, the bonding method and bonding
conditions employed, and, in appropriate cases, through further
simple experimentation.
Among the bonding methods which may be used in the practice of our
invention are included heat, electroplating, chemical bond
formation, chemical vapor deposition, plasma spraying,
thermosetting, dipping and drying in a solution of an organic
binder (i.e., structure forming agent; vide infra) and solvent
application of pressure to a mixed composite fiber network which
flows, melds, creeps, etc., or any other procedure which causes
physical attachment of all, or various types of selected, fibers
within the network.
Heating may cause similar or dissimilar metals or ceramics to
sinter via the atomistic diffusion of surface atoms so as to form
solder-like joints which provide good electrical and/or mechanical
contact. Alternatively, heating may cause dissimilar metals and
materials to overcome diffusive or reactive energy barriers
permitting surfaces of metalized or polymer-coated fibers to bond
at conditions different, or significantly less severe, than those
otherwise required to bond the base materials. The thermosetting
properties of polymeric or noncrystalline materials may also be
used to fuse these materials during an appropriate heat treatment
with the simultaneous application of an applied pressure force.
Electrodeposition of a metal (electroplating) into or onto the
mixed fiber composite provides a mechanism for growing or
thickening and strengthening contacts which are present or formed
between electrically conductive materials onto which the metal is
deposited. It also provides a mechanism for increasing the
electrical conductivity of the matrix, and, as noted earlier,
conductor "swelling" during electrodeposition can increase the
contact between conductive and nonconductive fiber materials.
Alternatively, electroplating via electroless deposition from a
metal salt and a suitable organic reducing agent can be used as an
indiscriminate bonding procedure which is operative regardless of
the base electrical conductivity of the material coated (e.g.,
aqueous silver nitrate plus formaldehyde produces a colloidal
suspension of reduced metal which bonds via precipitation and forms
adherent metallic films on various substrates).
Chemical vapor deposition and reactive plasma spraying provide
well-documented means of growing thin-films and coatings which have
the ability to coat, in a relatively uniform manner regardless of
geometry, various articles despite their dissimilar electrical or
mechanical properties. These procedures therefore have the ability
to bond similar or dissimilar materials which are in relatively
close proximity. Since the embodiment of this invention many times
involves mixtures of microscopic fibers in intimate contact, the
growth of a secondary deposit via, e.g., chemical vapor deposition,
can cause physical attachment of fibrous materials at locations
which had previously been in close proximity but not in direct
physical contact. This is an example where two fibers are bonded
via a bridge between them.
Dipping a composite fiber matrix in a solvated organic binder or
resin can cause attachment of similar and dissimilar fibers when
the solvent is removed by gradual drying and the binder is
concentrated via surface tension effects at the interstices and
intersections of the fibers (e.g., polyvinyl alcohol in water).
Subsequent drying leads to the physical attachment of the fibers
and high temperature carbonization or graphitization of the organic
can be performed to make the interconnecting material electrically
conductive. Alternatively, a solvated inorganic metal salt also can
be deposited upon drying at the intersections of the fibers with
this material being subsequently reduced to produce a conductive
coating and physical attachment.
Still another method of attachment might involve an ambient
temperature attachment of fibers through the application of
pressure exceeding that required to cause the material to flow via
plastic deformation. Such a process could be performed and/or
facilitated via the addition of an organic fiber or through the use
of a combined heat and pressure treatment.
In those cases where the first fiber is either a metal or a
ceramic, heating can be used effectively to sinter-fuse the fibers
at their junctions. The sintering temperature and sintering times
will vary greatly depending upon the nature of the materials to be
fused as well as the nature of the second fiber, but these
generally can be determined either via simple experimentation or
through knowledge of the activation energy of the various processes
which occur during sintering. For example, in the case where the
first fiber is stainless steel and the second fiber is carbon and
the sintering is done in the presence of hydrogen, then the
competing reactions are fusion of the stainless steel and
vaporization of carbon through reaction with hydrogen, especially
if catalyzed by the metal. Experimentation has shown that the
activation energy of the latter process is substantially less than
that of the former. In addition, the latter process is dependent on
hydrogen pressure. Therefore, the selectivity of sinter fusing the
stainless steel at its junctions may be optimized by heating under
low hydrogen partial pressure at relatively high temperatures for
relatively short times.
The particular bonding method as well as the conditions of bonding
will understandably depend upon the nature of the fiber materials
in the composite as well as its intended use. For example, where
the first fiber is a metal and the composite is intended for use as
an electrode, where good electrical contact between the metal
fibers is required, it is found that bonding via sintering is quite
effective. However, it also has been found that bonding at the
junctions is further improved by electroplating. The message we
wish to convey is that however significant may be the bonding
method in the general practice of our invention, the choice of the
particular bonding method used necessarily depends upon the nature
of the fibers in the composite as well as the intended use of the
composite.
Perhaps the most important subclass of composites is that where the
first fiber is a metal. Virtually any metal fiber may be used in
the practice of our invention, although generally the metal must be
chemically inert under the conditions of the contemplated use of
the composite and also generally must provide structural integrity,
strength, and mechanical stability to the final composite under the
contemplated conditions of use. For example, the final composite
generally needs to retain its overall shape and to remain
relatively rigid and immobile in most uses. However, where the
final composite needs to retain some flexibility in its operating
environment then materials need to be chosen which will impart such
properties. Illustrative but not exhaustive examples of metal
fibers which may be used in the practice of this invention include
aluminum, titanium, vanadium, chromium, iron, cobalt, nickel,
copper, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium,
palladium, silver, cadmium, indium, tin, hafnium, tantalum,
tungsten, rhenium, osmium, platinum, gold, antimony, berrylium,
iridium, silicon, magnesium, manganese, gallium, and combinations
of the above. Metal alloys also may be used in the practice of this
invention, as exemplified by constantin, hastelloy, nichrome,
inconel, monel, carpenter's metal, and various steels, especially
stainless steels, and other alloys. As can be appreciated, there is
enormous flexibility in the choice of metal fibers which adds to
the attractiveness of our invention.
The diameter of the metal fibers used is largely dictated by their
availability. Although in principle there is no upper or lower
limit to metal fiber diameter, there may be operational
restrictions in those cases where the second fiber is a non-metal
which is held together by the fused metal network. For example, if
the second fiber is a carbon fiber which is randomly intertwined
among fused metal fibers, then if the metal fiber diameter is
greater than ten times, or less than one-tenth, the carbon fiber
diameter the fused metal network may not hold the carbon fibers
together adequately. But in the more general case the ratio of
diameters of a metal first fiber to second fiber may range from as
high as 1000 to as low as 0.001, depending upon the nature of the
fibers, their density, and the intended use of the article, inter
alia. Another operational limitation may be related to the number
of bonded junctions which are largely responsible for supporting,
for example, the carbon fibers in the aforementioned composite.
Calculations show that the number of such junctions varies
approximately with the inverse of the square of the metal fiber
diameter, hence there is a requirement for small diameter metal
fibers where it is desirable to increase the overall weight
fraction of carbon or the other second fiber of the resulting
composite. But in the context of novel composites per se, the
diameter of the metal fiber used is not critical. The method of
preparation and attainment of the composite is not limited by metal
fiber diameter, at least up to about 50 microns. Metal fibers with
diameters as low as about 0.5 microns and with diameters up to at
least 25 microns have been used quite successfully in the practice
of our invention. It needs to be emphasized that the aforementioned
range is merely illustrative of the success which is to be
contemplated and is more representative of metal fiber availability
rather than being a limitation on the diameter of metal fibers.
Where the first fiber is a metal, the second fiber may be a metal,
a ceramic, carbon, a high surface area material, or any combination
of the above. One important subclass of composites results from the
second fiber being a metal. The metals which may be employed for
the second fiber constitute the same group of metals as may be used
for the first fiber as given above and need not be repeated here.
The second fiber may be a metal which is the same as or different
from that of the first fiber; that is, where the second fiber is a
metal it is independently selected from the same group of metals
from which the first fiber is chosen. Often the second fiber as a
metal will be distinguished by having a diameter different from
that of the first fiber. More particularly, relatively large
diameter fibers in a network impart strength and structural
integrity to the composites. On the other hand, a small diameter
second fiber may be chosen to adjust the void volume and porosity
of the resulting composite. Where the two metals used are of quite
different diameter, it has the effect of constructing a small mesh
network on a large mesh framework, which has been found to be a
very useful structure. The second fiber may have a diameter ranging
anywhere from about 0.001 that of the first fiber to 1000 times
that of the first fiber. The second fiber may be present at a
weight ratio of from about 0.001 to about 100 that of the first
fiber. It should be clear that adjustment of both the weight ratio
as well as the diameter of the second fiber enables one to control
the porosity and void volume of the resulting composite almost
without limitation and essentially continuously and enables one to
fabricate articles customized for their intended use. The ratio of
fiber length to fiber diameter, or aspect ratio, is yet another
independent variable, and in the variant where both the first and
second fibers are metals the aspect ratio of the second fiber can
range between about 10 and about 10,000.
As mentioned previously, where the composite is to be used as, for
example, an electrode and good electrical contact among the fibers
is paramount, then the fibers may be electroplated to enhance such
contact. This may be done both in cases where the first and second
fibers are metals, as well as in those cases where the second fiber
is not a metal, whether or not the second fiber is a conductor such
as carbon.
Another important class of composites is that where the second
fiber is not a metal but a high surface area material. Even though
the minimum surface area needed is 1.5 m.sup.2 /g, it is desirable
that such second fibers have a surface area of at least 50 square
meters per gram (m.sup.2 /g), although materials with a surface
area greater than about 100 m.sup.2 /g are preferred, and those
with a surface area greater than 250 m.sup.2 /g are particularly
preferred. Although there may be no theoretical maximum of surface
area which may be employed in the practice of our invention, as a
practical matter fibers with surface area over 1500 m.sup.2 /g are
difficult, if not impossible, to obtain. Among such high surface
area materials available as fibers are included carbon, silica,
magnesia, alumina, clays, titania, aluminosilicates,
silicaaluminophosphates, aluminophosphates, and so forth.
A particularly important subclass of composites of this invention
is a matrix of carbon fibers interlocked in and intertwined among a
network of fused metal fibers. Although it should be apparent that
"carbon" in the phrase "carbon fibers" includes and encompasses
graphite, we here specifically note that in the context of the
remainder of this specification and in the claims "carbon fibers"
includes graphitic material. The carbon fibers constitute from
about 1 to about 98 weight percent of the final composite, although
the range between about 20 to about 98 weight percent is preferred.
There is no significant upper or lower limit for the diameter of
the carbon fibers as regards forming the composite itself. That is,
the diameter of the carbon fibers used in the composite influences
its final properties rather than imposing limitations on whether
the composite itself can be made. Carbon fibers have been reported
with a surface area from about 1500 m.sup.2 /g to 1 m.sup.2 /g and
less, and with a diameter from 20 nm to about 1 mm. As an example,
and as will become clearer from the descriptions within, for use in
liquid double layer capacitors, H.sub.2 /H.sub.3 PO.sub.4 /O.sub.2
fuel cells, and Li/SOCl.sub.2 batteries, carbon fibers having a
surface area of from 250 m.sup.2 /g to about 1000 m.sup.2 /g are
most desirable with fibers having a diameter from 1 to about 10
microns, with a carbon content of the composite ranging from 30 to
about 90 weight percent.
The carbon fibers generally are present as bundles. Single fibers
tend to be brittle, whereas bundles or aggregates of fibers afford
a composite with more desirable mechanical properties. As the
diameter of the carbon bundles increases, the weight of metal
fibers needed to keep the bundles intertwined and interlocked is
decreased. The physical properties of the final composite also
depend on the physical properties of the carbon fibers used;
thermal stability, surface area, mean pore diameter, mechanical
flexibility, resistance to electrolytes and acids, and
electrocatalytic properties are examples of composite properties
which are influenced by the properties of the constituent carbon
fibers and any electroactive materials impregnated on the fibers.
It should be emphasized that the surface area of the carbon fibers
used largely determines the surface area of the final composite.
Since different applications require different characteristics, the
choice of carbon fiber properties often will be dictated by
composite application. For example, where used in double layer
capacitors one generally wants a certain minimum pore size, which
in turn limits the surface area. In batteries mass transfer is more
important and one wants a higher void volume, preferably with a
bimodal pore size distribution. A graded porosity also is possible
to attain using this invention and may be important in particular
applications. However, what needs to be emphasized is that many of
the composite properties are not only variable but are under the
control of the investigator or fabricator within quite broad and
flexible limits.
Where the composites are carbon fibers intertwined among, and
interlocked in, a network of bonded metal fibers, the metal fibers
which may be used in the practice of this invention must be
chemically inert under the conditions of their contemplated use,
and must provide structural integrity and mechanical stability to
the final composite under the contemplated conditions of use. So,
for example, the final composite generally needs to retain its
overall shape, and the metal of the composite serves to retain the
carbon fibers in the network relatively rigid and immobile. Where
the composite is used, e.g., in an electrode, the metal also must
be electrically conducting and its network must provide structural
and mechanical stability even in strong, electric fields. Because
of their general availability and relatively modest cost, as well
as favorable physical and chemical properties, various stainless
steels are the materials of choice, especially in many electrode
applications.
As previously stated, the method of preparation and attainment of
carbon fiber -metal fiber composites is not limited by metal fiber
diameter, at least up to about 50 microns. In the context of the
composite properties, however, the diameter of the metal fiber is
important. In practice it is desirable to have metal fibers with a
diameter under about 10 microns. It would be most desirable to use
metal fibers with a diameter in the range from about 0.5 microns to
about 4 microns, but it needs to be emphasized again that the
nature and diameter of the metal fibers used in the practice of
this invention are limited largely by their availability rather
than by any theoretical considerations.
The amount of metal in the final carbon fiber-metal fiber composite
depends on how much surface area per gram is important, and,
perhaps even more importantly, how good a contact is desired
between the metal and the carbon fibers. It should be clear that
the better the contact wanted, the higher the necessary percentage
of metal fiber (at constant fiber diameter) in the final composite.
Generally the composites of this invention will have a metal
content ranging from about 2 up to about 99 weight percent. As
metal content increases, the composite shows reduced resistance and
higher power density per gram with a lower surface area and lower
energy density per gram.
In yet another important group of composites the second fiber is a
ceramic material. For the purpose of this application a ceramic
material is an oxide, nitride, or carbide of metals such as
aluminum, titanium, vanadium, chromium, iron, cobalt, nickel,
copper, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium,
palladium, silver, cadmium, indium, tin, hafnium, tantalum,
tungsten, rhenium, osmium, platinum, gold, antimony, berrylium,
iridium, silicon, magnesium, manganese, gallium, and their
mixtures. Illustrative examples of ceramics which may be used in
our invention include silica, alumina, silica-aluminas, boron
nitride, boron carbide, silicon nitride, silicon carbide, titanium
nitride, titanium carbide, titanium boride, zirconium nitride,
zirconium carbide, niobium carbide, niobium nitride, molybdenum
nitride, molybdenum carbide, tungsten carbide, tantalum carbide,
and so forth. In that variant of our invention where the second
fiber is porous, for example, a high surface area material such as
carbon, alumina, or silica fibers, or some other porous material
including ceramics, it then may be impregnated with a metal or a
metal compound, especially one with catalytic properties for at
least one chemical process. This variant is especially suitable
where the second fiber has a relatively high surface area, as
mentioned above, and normally will be practiced in that mode.
As previously stated, any metal which exhibits suitable catalytic
properties may be used and are illustrated by metals such as
aluminum, titanium, vanadium, chromium, iron, cobalt, nickel,
copper, zinc, zirconium, niobium, molybdenum, ruthenium, rhodium,
palladium, silver, cadmium, indium, tin, hafnium, tantalum,
tungsten, rhenium, osmium, platinum, gold, antimony, berrylium,
iridium, silicon, magnesium, manganese, and gallium, as well as
their carbides, oxides, sulfides, nitrides, and combinations
thereof. The porous second fiber may be impregnated by the metal or
metal compound by any means known in the art. Such methods of
impregnation include electrochemical precipitation, infiltration
and drying, incipient wetness and drying, ion exchange, gas
adsorption, liquid absorption, and vapor deposition. Methods of
deposition are well known to those skilled in the catalytic art and
need not be further elaborated upon.
The composites of our invention can be made by a relatively
straightforward, uncomplicated method generally applicable to many
types of materials. The fibers, and other components where present,
are dispersed in a fluid medium along with an agent which we will
refer to as a structure forming agent. The resulting dispersion is
then cast into a predetermined shape and the cast dispersion is
treated according to the various bonding methods discussed earlier
so as to effect bonding of at least the first fibers at a plurality
of their junctions. Much, and often substantially all, of the
structure forming agent is then removed, often coincident with
whatever procedure is used to effect bonding, but sometimes done
sequentially, especially when the bonding procedure is followed by
chemical or electrochemical leaching of the structure forming agent
or pore former. As examples, residual materials can be removed
through the use of an appropriate and selective etching organic
acid; cores of electroplated carbon fibers can be removed via
thermal oxidation, plasma oxidation, etc.; a ceramic particulate or
void former can be preferentially removed by an appropriate
caustic. Our method is extraordinarily flexible and broadly
applicable as to the kinds of fibers which may be used in its
practice. It also exhibits virtual universality as to the resulting
shape of the finished article, and in fact net-shaped or near
net-shaped articles may be readily made. The method also manifests
some versatility as to bonding methods, which affords great
flexibility in processing procedures and thereby the types of
materials which may be used.
The fibers which may be used in this invention already have been
adequately discussed, obviating the need for further description.
These fibers are dispersed in a fluid medium along with other,
optional solid and/or liquid components. These optional components
are functionally significant in the final composite, becoming
entrapped and enmeshed in the network, and occupying what otherwise
would be the voids in the resulting composite. Examples of solid
particulate components which may be used in the practice of this
invention include zeolites, particulate catalysts generally,
adsorbents, ceramics, and combinations thereof.
The fibers and other components, if any, are dispersed in a liquid
by any suitable means. It is not essential to have a uniform
dispersion, although often such uniformity is desirable. Dispersion
may be effected by such means as sonication, agitation, ball
milling, and so forth. The purpose of the liquid is merely to
facilitate effective dispersion of the solids, especially where one
wants as uniform a dispersion as is feasible in the final preform.
Normally the liquid used will be unreactive with the other
components of the dispersion, but one can envisage special cases
where a functionally desirable reactive property of the medium may
be advantageously combined with its fluidity. Since the liquid is
later removed it is clear that it should be readily removable, as
by volatilization. Water is normally a quite suitable liquid,
although water-alcohol mixtures, and especially water-glycol
mixtures, may be used. Illustrative examples of other liquids
include methanol, ethanol, propanol, ethylene glycol, propylene
glycol, butylene glycol, poly(ethylene glycol)(s), poly(propylene
glycol)(s), and so forth. Other organic liquids also may be used,
but normally without any advantages. Since water is by far the most
economical and most universally available liquid it is the one of
choice in the practice of our invention. The liquid medium also may
contain salts where these are desirable, and the greater solubility
of salts in water relative to organic media also make the use of
water highly advantageous. While some mixtures of the above noted
liquids are used to adjust the viscosity of the dispersion so that
filtering or settling onto a screen or filter provides a certain
degree of uniformity within the "wet" preform regardless of the
densities and drag forces acting on the various particulates, still
other additives including surfactants and dispersing agents can be
used to assist in the mixing process and also to preferentially
associate at least two of the solids with one another in the
preform.
A preform is the solid containing a non-woven dispersion of the
fibers and any other optional components either in the structure
forming agent or located upon or within the pore volume of one or
more of the fibers. The structure forming agent provides a solid
matrix in which the fibers and optional components are dispersed.
The purpose of the structure forming agent is to permit the
fabrication of a solid preform of an otherwise structurally
unstable dispersion of the elements of the final composite where
the preform can be shaped, stored, and otherwise handled prior to
creation of an interlocked network via bonding of at least the
first fibers at their junctions. The structure forming agent merely
provides a stable, although relatively weak, physical structure
which maintains the spatial relationship of the components of the
final composite prior to the latter's formation. Although the
preform is only a temporary structure, it is an important one in
the fabrication of the final composite. The structure forming
agents used in the preparation of the preform also may contain
adjuncts such as pore and void formers.
A short comment on terminology may be in order. What we have called
"structure forming agent" is usually referred to as a "binder" in
other contexts. However more descriptive and more familiar "binder"
and "binding agent" may be, these terms might be confused with the
particular notion of "bonding" essential to the description of this
invention. It is solely to avoid such confusion that we adopt the
somewhat awkward term "structure forming agent."
The structure forming agents are chosen to volatilize at least to
the extent of 90 weight percent, more often to at least 95% and
frequently to at least 99 weight percent, under conditions which
are neither chemically nor physically detrimental to the fibers and
other components in the final composite (but vide infra). Where the
structure forming agent has no function in the composite which is
the rule rather than the exception, its presence can be minimal.
Among the structure forming agents which may be used in the
practice of this invention are cellulose, organic resins such as
polyvinyl alcohol, polyurethanes, and styrene-butadiene latex, and
thermosets such as epoxies, urea-formaldehyde resins,
melamine-formaldehyde resins, and polyamide-polyamine
epichlorohydrin resins. Cellulose, in all its forms and
modifications, appears to be quite desirable structure forming
agent because it volatilizes completely at relatively low
temperatures with little ash formation and is unreactive toward
other components in the preform.
The structure forming agent is present in the preform at a range
from about 2 to about 90 weight percent. The minimum amount of
structure forming agent is that which is necessary to give a stable
preform, that is, one which can be handled, shaped, and so forth,
an amount which depends upon fiber loading, fiber size, and so
forth. The amount of structure forming agent present in the preform
will influence the void volume of the final composite, with a
higher structure forming agent content affording a higher void
volume, hence the structure forming agent can be used as one
independent variable to control this property. We have previously
noted that where two metal fibers are used with different fiber
sizes, the amount of the smaller sized fiber also may be used to
vary void volume and pore size. Using cellulose as a structure
forming agent with carbon fibers and stainless steel fibers as an
example, a range from about 10 to about 60 weight percent of
cellulose in the preform is typical.
After the dispersion of fibers, optional components, and structure
forming agent in a liquid is attained, the solids are collected, as
on a mat. Excess liquid may be removed, such as by pressing, and
the resulting solid dispersion often is dried (i.e., liquid is
removed), especially where it is to be stored prior to further
treatment. Where a thermosetting structure forming agent is used,
the temperature of drying is important. But in the more usual case
there is nothing particularly critical in the drying process, and
drying may be performed in air, under elevated temperatures, or in
a flowing gas. The mass also may be compacted under pressure to a
greater or lesser extent to effect void volume; the greater the
compaction, the lower will be the void volume. This affords a third
independent means of controlling void volume in the final
composite. [A fourth independent means of controlling the void
volume and pore size can be effected by altering the tensile
properties and aspect ratios of the fibers used. A fifth means of
controlling void volume involves the application of pressure
during, e.g., sintering, or any other bonding procedure which may
be employed.]
The dispersion may be cast into a predetermined shape prior to,
coincident with, or after drying, with the last named procedure the
one most commonly employed. The preform resulting from drying is
generally quite flexible and adaptable to shapes of various sorts.
Often it is quite convenient to cast the dispersion into sheets
which can then be rolled up and stored prior to being treated to
effect bonding. The sheets can be stored for long periods of time,
can themselves be cast into near net-shaped bodies, and can be used
for onsite bonding procedures for the fabrication of various
articles. Various types of preform sheets may be stacked upon one
another prior to any treatment to effect bonding in order to create
thicker composites containing spatially graded compositions, graded
porosities, nonconductive separator functions, etc. Alternatively,
different shaped preform sheets may be stacked so as to form both
two and three dimensional structures for various applications.
Metal containing preforms can be, e.g., sintered onto thin metal
foils which serve as electrolyte barriers in the case of bipolar
electrode assemblies, or the metal foil may be omitted in the case
of flow-through geometries. More complex flow patterns and
geometries are also obtainable as cellulose-containing preform
sheets can be shaped and glued into "corrugated cardboard" like
structures prior to bonding treatment and have been shown to retain
their shapes after such treatment.
The preform or the cast dispersion is treated principally to effect
bonding of at least the first fiber junctions. The method used to
effect bonding often has the important secondary or ancillary
effect of removing the structure forming agent and the remainder of
the liquid medium. The removal of the structure forming agent and
the remainder of the liquid medium may be a second and discrete
step which either precedes or succeeds bonding. For economy of
exposition we will subsequently treat methods of bonding as
effecting concurrent removal of the structure forming agent and
remaining liquid, although it needs to be explicity recognized that
this is not necessarily the case.
Among the methods which may be used to bond at least the first
fibers at their junctions may be mentioned heating, electroforming,
electroplating, and various chemical reactions; the more complete
exposition of bonding methods given earlier should be consulted. At
least where bonding of metal junctions is sought to be effected,
heating is the most effective bonding method and also has the
desirable attribute of simultaneously effecting removal of some
types of organic structure forming agents and the remaining liquid.
Heating produces sintering of metal-metal junctions and also
ceramic-ceramic junctions, but is not necessarily effective with
other fibrous material. Another useful bonding method which may be
employed is electroplating. Other methods of bonding have been
described earlier.
In the preparation of carbon fiber-metal fiber composites, heating
the preform to effect sintering or fusion of metal-metal junctions
is the final stage in composite fabrication. The preform is heated
under conditions effecting sintering of the metals to provide a
network of fused metal fibers. Fusion of the metal fibers at their
points of contact rigidly locks the carbon fibers in place to
afford a rigid structure by defining a matrix of carbon fibers
intertwined and interlocked in a network of metal fibers with the
structural rigidity arising from a multiplicity of fused points at
the contact sites of metal fibers. Sintering typically is done in a
gas containing hydrogen at a partial pressure which is about 5
times the partial pressure of water in the gas stream, the water
typically arising from the binder and from oxides on the surface of
the metal. At the temperature of metal fusion the metal also
usually promotes gasification of carbon via its reaction with
hydrogen to afford methane. Consequently sintering preferably is
performed at a high temperature for a short time to promote metal
fusion relative to carbon gasification. It is desired that
sintering be accompanied by loss of less than about 25% by weight
of the carbon fibers via gasification, preferably under about 15%,
and even more preferably under about 5 weight percent loss.
Although the nature of the materials in the preform are important
to determine the particular fusion conditions, the relative amounts
of these materials are less important. The optimum sintering
temperature can be routinely determined by the skilled worker in
this field through simple experimentation. For example, where a
carbon fiber-stainless steel composite is obtained through a
preform with cellulose as a binder it has been determined that
fusion temperatures from about 1000.degree. C. to about
1200.degree. C. for a period from 2.5 minutes to 3 hours is optimum
in an atmosphere of H.sub.2 at 101 KPa. It may be noted in passing
that controlled void formation is a consequence of binder
volatilization.
It needs to be appreciated that although the foregoing temperatures
provide a workable range, the properties and composition depend on
the sintering time and temperature. Sintering at 1200.degree. C.
for 5 minutes produces an electrode material very different than
one formed using the same preform and sintering at 1000.degree. C.
for 3 hours. Depending on the specific application, either one
might be considered optimal. Certainly the corrosion resistance of
the metal and the metal/carbon ratio are greatly affected by the
sintering conditions shown.
Structure forming agents generally will be removed via methods
which will include volatilization (e.g., sublimation, evaporation,
oxidation to gaseous materials), carbonization, other chemical
reactions affording volatile products (or gasification generally),
acid or caustic leaching, and dissolution, whether dissolution of
the structure forming agent per se or of secondary products
resulting from chemical degradation or transformation of the
structure forming agent. Volatilization as by heating in a suitable
atmosphere is the most general method of structure forming agent
removal and is highly favored in the practice of our invention. As
noted above, the structure forming agent is removed at least to the
extent of 90%, in the more usual case at least 95% is removed, and
often at least 99% is removed.
The foregoing description was couched in terms of a structure
forming agent which was largely subsequently removed. In another
large class of composites the structure forming agent need not be
largely removed, and sometimes its removal is undesirable. For
example, one may employ as a structure forming agent a polymer
which subsequently undergoes carbonization but not volatilization.
The resulting composite is then a network of bonded fibers in a
graphitic matrix. As another example, in appropriate circumstances
it is possible to have a solvated metal salt as a structure forming
agent, which is later reduced to lock the structure together. What
is important to recognize is that however important may be the
class of structure forming agents which are largely removed by
subsequent treatment it is not the sole class of structure forming
agents which may be used in the practice of our invention.
As stated at the outset, the properties of our carbon fiber-metal
fiber composites may be varied over rather wide ranges. The surface
area of the composite depends upon the amount of carbon present as
well as the surface area of the carbon fibers used in its
preparation. It is desirable to have a composite with a high
surface area where the composite is used as an electrode, but with
a low surface area where the composite is used for electromagnetic
shielding. The surface area of the final composite may range from
about 0.001 m.sup.2 /g to at least 1350 m.sup.2 /g. In the general
field of electrochemistry, the most interesting range of surface
areas is from about 50 to about 1350 m.sup.2 /g, especially the
range 250-1000 m.sup.2 /g. The void volume of the composite
determines its ability, when used as an electrode, to accommodate
solid precipitates without affecting electrode surface area, and
the ability to provide good heat and mass transfer. Void volume, as
mentioned above, may be adjusted by the amount of the binder used,
as well as the diameter of the binder fibers and the application of
pressure during sintering. Clearly this is under the control of the
investigator who then has the capability of fabricating composites
with that set of properties desired for a specific application.
In the case of bipolar electrodes, required for liquid double layer
capacitors, Li/SOCl.sub.2 cathodes and H.sub.2 /H.sub.3 PO.sub.4
/O.sub.2 fuel cells, preform materials are placed on both sides of
a thin metal foil and sintered, as described earlier, so that the
metal fibers lock the high surface area carbon fibers to both sides
of the electrode foil. The metal foil serves as an electrolyte
barrier and an electrode base for connecting external contacts.
Metal fibers and the electrode base may be fabricated from the same
material, although dissimilar metals can be used provided highly
adherent and sinter-bonded contacts can be formed.
As stated earlier, the composites of our invention have a
multiplicity of diverse uses in addition to that of an electrode.
For example, the composite paper preforms can be stacked and
sintered with varying pore sizes, void volumes, etc., so as to form
tailored filter materials. These filter materials can be wrapped
around an appropriate mandrel so that near net shape properties are
obtained upon sintering. There does not appear to be any major
limitation on the fiber materials which are used. The independent
adjustment of pore size and void volume would help to make, e.g.,
stainless steel filters, which provide long lifetimes and lower
pressure drops prior to plugging.
Superconducting magnetic separators, with appropriate screen
materials, are routinely used in the minerals beneficiation
industry to remove magnetic ores and particulates from nonmagnetic
crudes. The force which attracts the magnetic particulate depends
upon a number of factors one of which is the magnitude of the
magnetic field gradient at the magnetic screen. Material holdup and
retention, and clogging prior to demagnetization with shaking and
rinsing also are design criteria.
In the past, methods have not existed for making screen materials
with independent optimization of void volume, pore size and fiber
diameter. Fiber diameter is important since the radius of the wire
and holes or voids in the resultant mesh control the magnetic field
gradient. Currently, 400 grade stainless steels with appropriate
magnetic properties are employed in these screens, but fibers below
10 or 20 .mu.m are generally not used since the screen or mesh
which is formed plugs easily due to the formation of small voids
and/or becomes weak when small diameter materials are employed if
the voids are kept large (viz., low density materials).
Our process (i) is directly applicable to 400-grade stainless
steels, (ii) can be used to achieve relatively independent control
of void volume and pore volume, (iii) can fuse small diameter loose
fibers into networks that are not available as freestanding
starting materials, (iv) can be used to form layered/stacked sheets
for graded porosities and enhanced performance and (v) can utilize
mixtures of both large and small diameter fibers. The latter
approach would permit larger fibers to be used for structural
support while zones of high magnetic flux gradient could be created
adjacent to these members using smaller diameter materials. Indeed,
the possibilities here seem endless.
In the case of bipolar electrodes, required for liquid double layer
capacitors, Li/SOCl.sub.2 cathodes, and H.sub.2 /H.sub.3 PO.sub.4
/O.sub.2 fuel cells, preform materials are placed on both sides of
a thin metal foil and sintered, as described earlier, so that the
metal fibers lock the high surface area carbon fibers to both sides
of the electrode foil. The metal foil serves as an electrolyte
barrier and an electrode base for connecting external contacts.
Metal fibers and the electrode base may be fabricated from the same
material, although dissimilar metals can be used provided highly
adherent and sinter-bonded contacts can be formed.
The experimental description and results which follow only
illustrate this invention and are representative of the methods
which may be used and the results which may be obtained, but should
not be considered as limiting the invention in any way.
EXPERIMENTAL
The following description is representative of the preparation of
the composites prepared within. Differences in materials,
conditions, etc., will be indicated for the individual composites
where appropriate.
Materials-The constituent materials employed during composite
preparation included carbon fibers from Charcoal Cloth, Ltd., 316L
stainless steel fibers from Bekaert Steel Wire Corp. and/or
National Standard, cellulose fibers as a mixture of soft and hard
woods, and 316L stainless steel foils from Arnold Engineering.
Individual carbon fibers were 2-3 microns in diameter but were used
in the form of 10 micron diameter bundles up to 5 mm in length
containing ca. 30 individual fibers. Cellulose fibers were 20-30
microns in diameter and varied in length from 100 to 1000 microns.
The stainless steel foils were 5 microns in thickness.
Fiber preparation-Before the various fiber materials could be
combined into a paper preform, the carbon and stainless steel
fibers required separation and dispersion into a slurry for easy
mixing with other materials. In raw form, the carbon fibers were
bundled and twisted into strands and woven into charcoal cloth. The
"cloth" was dismantled into strands, then cut into 0.5 cm sections
to allow for dispersion of individual fiber bundles in water. "As
received" stainless steel fibers were coated with polyvinyl alcohol
(PVA) type Mowiol 4-88, which was utilized during sizing and
cutting prior to shipment. PVA was removed by repeated rinsing of
these fibers in distilled water.
Formation of paper preform-Since physical mixtures of the fibers
are not mechanically stable, cellulose fibers were employed as a
structure forming agent in the preparation of paper preforms. The
paper preforms used in composite preparation were processed
according to TAPPI Standard 205 using Noran equipment. The
pretreated fibers along with cellulose fibers were agitated at 50
Hz in 1 liter of water for five to twenty minutes. The dispersed
fiber mixture was then collected on a sheet mold (200 cm.sup.2) to
form the wet paper composite preform. The preform was pressed at
ca. 400 kN/m.sup.2 and allowed to dry in air at room
temperature.
Assembly of electrode preforms-The first fiber-second
fiber-cellulose composite papers (i.e., paper preforms) and
stainless steel foils were cut into circular disks with diameters
of 13 and 19 mm respectively, and assembled by layers. In most
cases, an optional 19 mm diameter sheet of stainless
steel-cellulose paper preform was placed on top of each side of the
composite structure to serve as a protective layer, as shown in
FIG. 2.
Sintering of electrode preforms-The layered electrode preform was
placed between two quartz plates (20.times.30 mm), which were held
in place by a quartz clip. The sample was placed in a controlled
atmosphere quartz U-tube reactor (25 mm diameter) for heat
treatment. The sintering reactor was equipped with flexible gas
lines to facilitate movement of the reactor into and out of the
vertical sintering furnace (Heviduty, 10 A, 1150 W). Sintering was
performed in a reducing atmosphere of H.sub.2 with a flow rate of
10-100 cc/min (STP) and total pressure of 101 kPa. Gases were
supplied by Liquid Air with purities of 99.995% for H.sub.2. Gas
flow was monitored using a Linde Model FM-4550 flow controller.
The feed gas mixture was passed over Cu turnings at ca. 500K. to
remove background CO, CO.sub.2, O.sub.2 and H.sub.2 O and then
passed through a molecular sieve trap immersed in a liquid N.sub.2
trap to further remove background condensibles. The sintering
reactor was passivated with feed gas for a minimum of three hours
prior to reaction. The sintering furnace was preheated to 1423K.
prior to beginning each experiment. The reactor was then introduced
into the furnace causing a rapid cooling of the furnace to ca.
1400K. The experimental temperature was typically reached in 5-7
min followed by sintering at the desired temperature. The sintering
reactor was quenched by rapidly removing it from the furnace.
Sample analysis-The amount of carbon retained in the carbon
fiber-stainless steel fiber composite electrode after sintering was
estimated from weight change measurements. These measurements were
obtained on a Sartorius Model R 160 D semimicro balance with a
precision of 0.02 mg.
Volumetric N.sub.2 B.E.T. surface area measurements were performed
to determine whether the high surface area characteristics of the
carbon had been retained. Measurements were taken of virgin
charcoal cloth before paper perform preparation and of composite
electrodes after sintering. The B.E.T. apparatus employed was a
high-vacuum Pyrex design with a base pressure of 4*10.sup.-2 Pa. To
minimize background impurities, high-vacuum greaseless stopcocks
(Ace Glass) were used to manipulate gas storage and dosage.
Experimental pressures were monitored within 1.3 Pa using a Texas
Instruments precision manometer (Model 145) employing a
fused-quartz Bourdon capsule. Samples were pretreated by heating in
vacuum at 473 K. for a minimum of 2 hrs to remove species such as
water from the sample. For each experiment performed, a minimum of
four data points were collected over the pressure range of 5.1 to
30 kPa.
The surface compositions of stainless steel foils in the sintered
composite were determined using X-ray photoelectron spectroscopy
(XPS). XPS analysis was performed using a Leybold-Heraeus LHS-10
spectrometer utilizing MgK.alpha. X-rays. The sample was exposed to
air for ca. 100 hours before measurements were performed, allowing
the surface to oxidize. Analysis was performed at 300 K. under a
vacuum of 1.3*10.sup.-6 Pa. Surface compositions were calculated on
the basis of measured peak area ratios normalized with respect to
the appropriate cross sections, inelastic electron escape depths,
and spectrometer sensitivity factors.
Scanning electron microscopy (SEM) was utilized to observe the
degree of intermixing of the constituent fibers and sintering
behavior. SEM micrographs were collected on an ISI Model 5540
scanning electron microscope at 5 kV beam energy.
EXAMPLE 1
CARBON RETENTION AT VARIOUS SINTERING CONDITIONS
Preforms were made from stainless steel fibers 2 microns in
diameter and 2 mm in length, and the aforedescribed carbon and
cellulose fibers under the general preparative conditions stated
above. The air-dried preforms were heated at the temperatures of
Table I to determine carbon retention and sintering degree.
The amount of carbon retained in the sintered composite matrix was
determined by weight change measurements with the assumption that
the metal weight would not change during sintering and that all
cellulose would be converted to gaseous products. Separate
experiments verified that the weight of retained cellulose after
exposure to hydrogen at 1323 K. was negligible. At these
conditions, the weight change of stainless steel was not
detectable. Based on carbon retention measurements, "optimal
sintering" determined by the percentage of initial carbon remaining
in the sintered electrode, was achieved at 1423 K. in H.sub.2 for
2.5 minutes. Gas flow of the H.sub.2 was maintained at 10 cc/min
(STP) with a total pressure of 101 kPa. For these optimal sintering
conditions, carbon retentions of >98% were attained. Results of
selected sintering experiments are shown in Table I.
TABLE I ______________________________________ Carbon Retention as
a Function of Sintering Conditions Percentage of Temperature Time
Initial Carbon Degree of Experiment (K) (min) Retained (%)
Sintering ______________________________________ A 1323 10 97.3 G B
1323 5 ND NS C 1373 5 97.5 G D 1373 2.5 ND NS E 1423 2.5 98.3 G F
1423 1.5 ND NS ______________________________________ G Good,
appeared structurally stable ND Not Determined NS Not Sintered, no
structural integrity
B.E.T. surface area-Of equal importance to the retention of carbon
is the requirement that carbon retains its high surface area
structure after sintering. Volumetric B.E.T. measurements showed a
surface area of ca. 760 m.sup.2 /gm of carbon for the sintered
composite electrode structure compared to ca. 790 m.sup.2 /gm for
virgin charcoal cloth.
Surface composition-XPS measurements of stainless steel foils that
had undergone sintering at the conditions of Experiment F in Table
I showed that iron was the most abundant metallic surface species.
The surface abundance of iron, chromium, and nickel were
investigated. The peak shapes and locations obtained are consistent
with those reported for iron (+3) oxide (Fe.sub.2 O.sub.3) and
chromium (+3) oxide (Cr.sub.2 O.sub.3) (21). Fe.sub.2 O.sub.3 was
found to be 1.8 times more plentiful than Cr.sub.2 O.sub.3 on the
surface. No nickel oxide (NiO) was detected. Results for the bulk
and surface compositions of sintered 316L stainless steel foils are
presented in Table II.
TABLE II ______________________________________ Bulk and Surface
Composition of Stainless Steel Type 316L Foil. Bulk Surface.sup.1
Heat of Compostion Composition Sublimation (atomic %) (atomic %)
(kJ/mol) ______________________________________ Chromium 17 35 396
Iron 71 65 416 Nickel 12 ND 429
______________________________________ ND Not Detected by XPS
.sup.1 Based on observed metal content only
Composite matrix structure-The degree of intermixing of the fibers
in the composite electrode matrix was investigated using SEM. FIGS.
3 and 4 show micrographs of the stainless steel-cellulose and
stainless steel-carbon-cellulose composite paper preforms,
respectively, prior to sintering. The degree of intertwining of the
metal and the carbon fibers in the two paper preforms is clearly
shown in the micrographs.
FIG. 5 shows the metal-carbon composite matrix after sintering
following the conditions of Experiment E in Table I. No cellulose
appears in the structure and the intertwined and interlocked
framework of the sintered composite is evident. The intimate
contacting of stainless steel and carbon fibers can be seen in FIG.
6. FIG. 7 shows the degree of metal-metal sintering which occurs in
the sintered matrix. This sintering appears responsible for the
electrode's outstanding structural integrity and electrical
conductivity.
EXAMPLE 2
A mixed-fiber composite was made from 2 .mu.m diameter and 0.5
.mu.m diameter 316 stainless steel (ss) fibers combined in equal
weight fractions. The length of the 2 .mu.m fibers added to the
preform were 5 mm, the length of 0.5 .mu.m fibers were ca. 100
.mu.m. Electrodes were prepared by casting a 16 cm diameter,
circular preform sheet, using 0.5 g of 2 .mu.m diameter 316
stainless steel fibers, 0.5 g of 0.5 .mu.m diameter 316 stainless
steel fibers, and 0.5 g of cellulose fibers. The fibers were mixed
at 50 Hz agitation in 1 liter of water prior to settling onto a
filtration screen. The preform sheet was pressed at 400 kN/m.sup.2,
dried in air for >24 hours and sinter bonded at 1323K for 20
minutes in 101 kPa of H.sub.2.
EXAMPLE 3
The electrode of FIG. 8 was prepared by casting a 16 cm diameter
circular preform sheet, 0.5 g of 2 .mu.m diameter 316 stainless
steel fibers, 0.5 g of 0.5 .mu.m diameter 316 stainless steel
fibers, 0.5 g of cellulose fibers and 0.5 g of a commercially
available alumina-silica fiber, Saffil, obtained from ICI
Chemicals. Saffil is 95% alumina and 5% silica, and fibers are
3.+-.1 .mu.m in diameter with a surface area of 150 m.sup.2 /g. The
length of the 2 .mu.m steel fibers was 5 mm, the length of the 0.5
.mu.m ss fibers was 100 .mu.m. The fibers were mixed at 50 Hz
agitation in 1 liter of water prior to settling onto a filtration
screen. The preform sheet was pressed at 400 kN/m.sup.2, dried in
air for >24 hours and sinter bonded at 1323K for 20 minutes in
101 kPa of pure H.sub.2.
EXAMPLE 4
The composite of FIG. 9 was prepared by casting a 16 cm diameter
circular preform sheet using 0.5 g of 2 .mu.m diameter 316
stainless steel fibers, 0.5 g of 0.5 .mu.m diameter 316 stainless
steel fibers, 0.5 g of cellulose fibers, and 0.5 g of a filler clay
for fibers (Kaolinite with mica particles; Hi White, available from
Huber Clays Inc.) The length of the 2 .mu.m ss fibers was 5 mm, the
length of the 0.5 .mu.m ss fibers was 100 .mu.m. The preform sheet,
prepared as described in the earlier examples, was pressed at 400
kN/m.sup.2, dried in air for >24 hours and sinter bonded at
1323K for 20 minutes in 101 kPa of pure H.sub.2.
EXAMPLE 5
The composite of FIGS. 10 and 11 were prepared by casting a 16 cm
diameter, circular preform sheet, using 0.5 g of 2 .mu.m diameter
316 stainless steel fibers, 1.0 g of 0.5 .mu.m diameter 316
stainless steel fibers, 2.0 g of cellulose fibers and 2.5 g of a
commercially available biosupport of calcined mullite known as
Biofix available from English China Clays. The length of the 2
.mu.m ss fibers was 5 mm, the length of the 0.5 .mu.m ss fibers was
100 .mu.m. The liquid used was water combined with a cationic
retention aid obtained from Betz Paper Chemicals which assisted the
biosupport in associating with the cellulose fibers while in an
aqueous solution. The preform sheet was prepared, pressed, dried,
and sintered as described in the prior example. FIG. 12 is an
electronmicrograph of material prepared by circulating a solution
containing yeast cells through the composite described above. The
cells shown in the Figure have become entrapped within the matrix
and the resulting composite can now be used, for example, as an
enzyme reactor, or for fermentation employing a steady-state cell
population.
EXAMPLE 6
A composite was prepared by casting a 16 cm diameter circular
preform sheet, using 0.5 g of 2 .mu.m diameter 316 stainless steel
fibers, 1.0 g of 2 .mu.m diameter carbon fibers and 0.5 g of
cellulose fibers. The length of the 2 .mu.m ss fibers was 5 mm. The
carbon fibers were cut to a length of 5 mm and had a surface area
of ca. 800 m.sup.2 g. These fibers were left in the form of ca. 10
.mu.m bundles containing ca. 30 fibers per bundle. The preform
sheet was prepared, pressed, dried, and sintered as described in
the prior example.
EXAMPLE 7
A composite was prepared by casting a 16 cm diameter circular
preform sheet, using 0.5 g of 0.5 .mu.m diameter 316 stainless
steel fibers, 1.0 g of 2 .mu.m diameter carbon fibers described in
the prior example, and 0.5 g of cellulose fibers. The length of the
0.5 .mu.m ss fibers was 1000 .mu.m. The preform sheet was prepared,
pressed, dried, and sintered as described in the prior example.
EXAMPLE 8
A commercially available electrode material, Protech, available
from Electrosynthesis Inc. of East Amherst, N.Y., was purchased as
a 24 mg/cm.sup.2 sheet containing 10 weight percent supported Pt
crystallites and a porous Teflon separator material as a backing
and used as the commercial reference.
Our material labeled as "Composite" was prepared by casting a 16 cm
diameter, circular preform sheet, using 1.0 g of 2 .mu.m diameter
316 stainless steel fibers, 1.0 g of 2 .mu.m diameter carbon fibers
and 0.5 g of cellulose fibers. The length of the 2 .mu.m ss fibers
was 5 mm. The preform sheet was prepared, pressed, dried, and
sintered as described in the prior example.
To produce the electrode of FIG. 13, one piece of the preform sheet
prepared above was cut into a 1.3 cm diameter circular piece and
sandwiched between two 1.9 cm diameter pieces of a masking and
protective preform which contained only stainless steel and
cellulose fibers. The sheet for the protective preform was prepared
identically to the preform sheet for the active layer except that
the carbon fiber bundles were omitted and the mass of stainless
steel fibers and cellulose fibers per sheet were both 0.5 g. The
resulting stack of three sandwiched preform pieces was sinter
bonded at 1323K for 30 minutes in 101 kPa of pure H.sub.2 to afford
an article of density 16 mg/cm.sup.2 which contained no Pt. The
stainless steel layers on each side of the electrode served as a
protective layer to protect the carbon-containing layer from any
type of mechanical abrasion.
The data of FIG. 13 are a comparison of polarization data between
our electrode and the commercial product. At low current densities
the commercial product is superior. However, at higher current
densities our electrode shows less polarization losses, even though
our material does not contain Pt. The open structure of our
material permits greater mass transport than the commercial
material. The low void volume and porosity of the commercial
product does not allow its active Pt materials to participate in
the reaction at high reaction rates, whereas the inherent activity
of the carbon, when accessible, is more than enough to overcome the
presence of Pt at high reaction rates, as demonstrated by our
electrode material.
EXAMPLE 9
The material described in Example 6 was subsequently electroplated
with Ni from a NiSO.sub.4 -6H.sub.2 O solution at the indicated
current densities and times. The 21 weight percent carbon in the
sintered electrode, rather than the expected 67% from the preform
composition, reflects the fact that the preform was sintered onto a
316 ss foil and that some of the carbon was gasified during
sintering. Performance data are summarized in Table III.
TABLE III ______________________________________ Nickel
Electrodeposition into Composite Electrode
______________________________________ Amount of Carbon in 0.00582
gm (20% carbon by weight) Composite electrode: Geometric Electrode
area: 1.27 cm.sup.2 Plating Solution: 0.2 M NiSO.sub.4.6H.sub.2 O
Counter Electrode: Platinum mesh Reference electrode: SCE Applied
Current Density Time Held Double Layer (mA/cm.sup.2) (mins)
Capacitance (F/g) ______________________________________ -- -- 1.79
27.6 10 2.61 55.1 5 2.80 78.7 5 2.90 157.5 6.7 2.85 236.0 6.7 0.395
275.0 6.7 0.347 ______________________________________
The column marked "Double Layer Capacitance" is based on the weight
of carbon only and does not include contributions from the ss foil
or ss fibers. The increasing values in this column indicate how
electrodeposition initially causes greater contact between carbon
and ss, while at higher current densities and longer times the
decrease is due to a blockage of the active material by a nickel
overlayer, with a sharp decrease in capacitance occurring at an
applied current density over about 160 mA/cm.sup.2. Overall, this
material maintains its high and accessible surface area at 15 to 27
times higher current densities than current commercial
cathodes.
* * * * *